An electrical power generator converts mechanical energy into electrical energy. A typical electrical power generator includes a stator and a rotor that rotates within the stator to thereby generate electricity. The rotor, in turn, is mounted to a shaft that drives the rotor. Various mechanical devices may be used to drive the shaft, such as a combustion turbine.
In a conventional configuration, the combustion turbine comprises a compressor to draw in and compress a gas, such as air, for example. The combustion turbine may also comprise a combustor or heat source that adds energy to the compressed gas, and a turbine to extract power from a resulting heated gas expansion. In an electrical generator, the extracted power is used to drive the shaft, which, as noted above, rotates the rotor within the stator to generate electricity.
The capacity of the combustion turbine may be increased if air drawn in by the compressor has a lower temperature relative to that which it will attain during combustion to drive the turbine. Accordingly, cooling the ambient, or inlet, air before it is drawn into the combustion turbine can be a cost effective way to increase the capacity of the combustion turbine. One approach to cooling the inlet air is with a direct refrigeration-cooling system in which ambient air is cooled using conventional refrigeration devices and techniques. One drawback to the direct refrigeration-cooling system, however, is parasitic power loss. This is due to the relatively large power drain needed to power a refrigerator unit. According to some estimates, the parasitic power loss may be as much as thirty percent (30%) of the increased power output of the turbine power generator.
An alternative inlet air cooling technique is provided by an evaporative cooling or fogging system. With such a system, moisture in the form of a water mist or spray is added to the inlet air. As the water evaporates, the temperature of the inlet air is lowered to a new temperature, thereby cooling the inlet air before it is drawn into the compressor of the combustion turbine. An evaporative cooling system tends to be less expensive to install and to operate as compared with other techniques and devices.
Despite the advantages of evaporative cooling, measuring inlet air temperature may be made more difficult by an evaporative cooling system, which includes dripping water on media, fogging systems, spraying, and other methods, as understood by those skilled in the art. Air inlet temperature typically has been measured in conventional combustion turbine power generators that lack an evaporative cooling system by using various temperature-sensing devices, including thermistors and thermocouples. As disclosed, for example, in U.S. Pat. No. 5,252,860 to McCarty et al. and U.S. Pat. No. 5,103,629 to Mumford et al., air temperature can be measured using a thermocouple positioned adjacent a compressor air inlet.
Similar temperature sensing devices have also been employed with combustion turbine power generators that use evaporative cooling. U.S. Pat. No. 5,930,990 to Zachary et al., for example, discloses an apparatus for adding water to a gas turbine. The water is added from a spray rack assembly comprising at least one water pipe and at least one corresponding water nozzle. The water is added through a duct that helps direct the water to a compressor inlet. Inlet air temperature is measured with a temperature sensor that, as illustrated, extends into the air flow path to which water has been added.
A problem associated with conventional temperature sensing devices when used in combustion turbine power generators that use evaporative cooling is that temperature readings may be adversely affected. More specifically, water droplets may form on an exposed temperature sensing device when water is added to the inlet air. Evaporation of the water droplets on the temperature sensing device may cause an inaccurate, lower temperature reading.
This tendency is especially problematic in combustion turbine power generators using evaporative cooling since the amount of water that should be added is a function of the temperature of the inlet air. Inaccurate temperature readings may cause an inappropriate amount of water mist to be added to the inlet air resulting in premature erosion of compressor blades.